Text 23

Thermal processes. Thermal cracking

Thermal processes

At high temperatures, the bonds between atoms in molecules of hydrocarbons are weakened and can break to form new compounds. In any homologous series, lighter [low-boiling] hydrocarbons split less easily than high-boiling ones. Along with splitting into lighter hydrocarbons, other transformations can take place, in particular, packing of molecules in which heavier fractions from preliminary petroleum processing are decomposed at elevated temperatures are call thermal processes. In petroleum processing industry, the most common processes of this type are thermal cracking, coking, and pyrolysis.

Thermal cracking, usually carried out at pressures up to 5 MPa and temperatures of 420-550 0C, is a process in which the starting material is changed qualitatively with the formation of new compounds having different physicochemical properties. Depending on the composition of the starting material and the process conditions, the yield of gasoline cracking is 7-30 % of the mass of the starting material; the process also gives some other products: gaseous, liquid and solid [coke].

Coking of residue is done at temperatures of 445-5600C [still coking] or 485-5400C. Depending on the quality of the starting material and the type and conditions of the process, it may yield 15-18 % of commercial coke, 49-77.5 % of liquid products [including 7-17 % of gasoline fractions] and 5-12 % of gases [up to C4].

Pylolysis of distillates and light hydrocarbons [from ethane to butane] is usually effected at 650-8500C. The main object of pyrolysis is to produce ethylene and propylene; earlier, it was aimed at producing aromatic [benzene] hydrocarbons.

In 1930-1950's, pyrolysis played an important part as a method for increasing the manufacture of gasolines for carburettor engines. At a later time, the quality of gasolines produced in thermal cracking plants could no more satisfy the rising requirements of consumers. Upon development of catalytic processes, thermal cracking still retains its role mainly for the manufacture of low-viscous fuel oils from residue products of preliminary petroleum processing , and also of gas oils intermediate products for making carbon black. The processes of coking are being developed further, mainly to satisfy the demands for coke, especially electrode coke. Liquid products of coking are utilized for increasing production of white petroleum products. Pyrolysis is being developed rapidly in association with increasing demands for olefin materials for the chemical and petrochemical industries.

Thermal Cracking

In 1890, V.G. Shukhov, a famous Russian scientist, designed the first cracking plant for producing light petroleum products from fuel oil. Later, as the need for automobile gasoline increased, a system with reaction chambers was developed, in which the starting material, preheated to the reaction temperature in the furnace coil, was retained and subjected to cracking up to the formation of coke. The time of filling of the reactor with coke determined the length of the whole working cycle of the plant. At a later time, the reaction chamber was replaced by the reaction volume formed in radiant pipes of a furnace. To prevent the clogging of the apparatus with coke, the reaction products were chilled at the exit from the furnace by the cold starting material [quench] which stopped the cracking process [in particular, Winker-Koch plants operated by this principle]. In later years, further improvements have been made in thermal cracking in foreign countries and in the USSA where the process was implemented in 1927-28.

As has been given earlier, the principal reaction of thermal cracking is the decomposition [or cracking ] reaction. Among various hydrocarbons, paraffins can be cracked most easily. Then follow naphthenic hydrocarbons. Benzene hydrocarbons are most stable against cracking. In any homologous series, hydrocarbons of a higher molecular mass are cracked more readily. Thus heavier fraction of petroleum products are less stable and can be cracked more easily than lighter ones. Brief data on the chemistry and mechanisms of cracking of the principal classes of hydrocarbons will be given below.

Paraffin hydrocarbons. Cracking of commercial paraffins which consist mainly of C24H50, C25H52 and C26H54 hydrocarbons forms paraffin hydrocarbons and olefins composed of 12, 13, or 14 carbon atoms, i.e. roughly one-half of the carbon atoms in the original paraffin. This is an indication of that the breakdown of C-C bonds in cracking of paraffins of high molecular mass occurs in the middle of a molecule. The new paraffin hydrocarbons formed by cracking can in turn break down into simpler molecules say a molecule of a paraffin hydrocarbon and that of an olefin, for instance:

4250C

C12H26  C6H14 + C6H12

dodecan hexane hexene

[paraffinic] [paraffinic] [olefinic]

At higher temperatures of cracking of paraffinic hydrocarbons, reactions in which the breakdown of molecules occurs at the end portion of the chain begin to prevail over those in which molecules break in the middle. The larger fragment of a broken molecule is an olefin and the smaller one is the paraffinic hydrocarbon [gaseous] or hydrogen. Isoparaffinic hydrocarbons are thermally less stable than those of the normal structure. The rate of the reaction at a given temperature increases almost linearly with the molecular mass. This is true of all groups of hydrocarbons.

Olefinic Hydrocarbons. These are the principal ones among all unsaturated hydrocarbons produced by cracking. They prevail as gaseous compounds [from ethylene C2H4 to butylene C4H8] and liquid ones [from amylenes C5H10 to pentadecenes C15H30]. Cyclic olefins and diolefins form in relatively small quantities. In contrast to paraffinic hydrocarbons, olefins undergo appreciably more diverse primary reactions during cracking, the most important among them being polymerization reactions [i.e. combination of a few molecules into a single molecule] and depolymerization reactions, especially at an early stage of the process . Polymerization is the main reaction at moderately high and high pressures; it can occur not only between like molecules, but also between unlike molecules of olefins, for instance:

C2H4 + C3H6  C5H10

At later stages of the process, olefins are dehydrogenated partially and form diolefins, which typically have two double bonds, and hydrogen or split into diolefins and paraffinic hydrocarbons:

CH3- CH2- CH= CH2  CH2= CH- CH= CH2 + H2

butylene divinyl

[olefin] [diolefin]

Secondary reactions between olefins and diolefins may give cycloolefins which are present in cracking products in very small quantities. Olefins can transform into cyclic hydrocarbons [naththenes]:

n-hexene -1  cyclohexane

Naphthenic hydrocarbons. The main reaction in cracking of these hydrocarbons are dealkylation [splitting of paraffinic side chains] and dehydrogenation of hexacyclic naphthenic hydrocarbons into benzene hydrocarbons; the two reaction can occur simultaneously.

Dehydrogenation of hexacyclic naphthenes in thermal cracking with the formation of benzene hydrocarbons is of minor importance. Owing to the dealkylation reaction taking place in thermal cracking, naphthenic and benzene hydrocarbons loss most of their long side chains. Paraffinic side chains in turn break to form gaseous and low-boiling paraffinic hydrocarbons and olefins. In high-temperature processes, naphthenic rings can break; the result is that hydrocarbons lose their cyclic structure and that polycyclic structures are partially decycled [if they had several rings]. In that case, paraffinic, olefinic and naphthenic hydrocarbons form.

Benzene Hydrocarbons. These are obtained by dehydrogenation of the cycloolefins or naphthenes which were formed at earlier stages of the process. Benzene hydrocarbons are quite stable at high temperatures, especially benzene, toluene and xylenes. The main reaction in cracking of benzene hydrocarbons with alkyl chains are dealkylation and condensation. Condensation may occur between the molecules of benzene hydrocarbons [or some other unsaturated hydrocarbons]. This gives polycyclic benzene hydrocarbons which can condense further to asphaltenes and coke.

Sulphur compounds. They are decomposed in cracking and form hydrogen sulphide. Cyclic sulphur-organic compounds, such as thiophene and thiophane, have the greatest stability against decomposition. Hydrogen sulphide and elemental sulphur [as the product of oxidation of hydrogen sulphide] which form in cracking of sulphurous petroleum grades can cause strong corrosion of process equipment.

Inert tars and asphaltenes. These may contain various heterocyclic compounds [usually including oxygen, sulphur, nitrogen and some metals]. In cracking they form gases, liquid products and large amount of coke. The yield of coke in cracking of asphaltenes may reach 60% and that in cracking of tars 7-20% [depending on the molecular mass of tars].

Since the starting materials for industrial thermal cracking are usually mixtures of many hydrocarbons of complicated structure, many reaction can occur simultaneously and the mechanism of thermal cracking can not be explained in detail. It is assumed however, that most reaction of thermal cracking can be described by the theory of formation of free radicals.

Exercises

Answer the following question

1. What are thermal processes ?

2. What are the most common processes of thermal processes ?

3. What are the products of thermal cracking ?

4. What are the products of coking?

5. What are the products of pyrolysis?

6. Who designed the first cracking plant?

7. Which type of hydrocarbon can be cracked most easily?

8. Which C-C bonds are broken down in cracking of high paraffins at lower temperature ?

9. Which C-C bonds are broken down in cracking of high paraffins at higher temperature ?

10. What are the products of cracking of high molecular mass paraffins at higher temperature ?

11. Which relationship is there between the rate of a reaction and its temperature?

12. What are the primary reaction of olefins in thermal cracking condition?

13. What are the secondary reaction of olefins in thermal cracking condition?

14. Which reactions happen with naphthenic hydrocarbons in thermal cracking condition?

15. Why are gaseous, low-boiling parafinic hydrocarbons and olefins formed in thermal cracking of naphthenic hydrocarbons ?

16. What are the main reactions of benzene hydrocarbons ?

17. Which compounds can be obtained in cracking of benzene hydrocarbons ?

18. Which sulphurous compounds are formed in cracking ?

19. What is the main product in cracking of tars and asphaltenes?

20. What is the main mechanism of thermal cracking ?